The influence of body temperature on MAC of halothane in the rabbit determined using controlled mechanical and heat stimulation

Author:

Sobair, A. T. H.

Awarding Body:

University of Edinburgh

Current Institution:

University of Edinburgh

Date of Award:

1995

Availability of Full Text:

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Abstract:

The determination of the minimum alveolar concentration (MAC) of any inhalation anaesthetic requires repeated application of the stimulus. The investigation of the influence of body temperature on MAC in the same animal, requires even more applications of the stimulus. The use of what is commonly known as a "supramaximal stimulus", beyond which there is no increase in MAC with any further increase in stimulus intensity, is generally held to provide for the reproducibility of MAC. However, there are many conflicting reports with regard to the MAC values obtained using "supramaximal stimuli" and with the interpretation of results. In this study the unreliability of the "supramaximal stimulus", in the form of tail clamping using a haemostat for the determination of MAC of halothane in the rabbit, was established. The use of this technique was demonstrated, both grossly and histologically, to cause severe trauma and lacerations that may have altered the sensitivity of the sensory mechanisms as indicated by the great variability in the values of MAC obtained on subsequent determinations in the same animal. Also, the technique appeared to disregard modern physiological concepts on receptor thresholds, inflammation and hyperalgesia. Therefore, it was decided to explore more reliable techniques of stimulation. Controlled mechanical and heat stimulators were devised and used for the determination of MAC of halothane in the rabbit as the body temperature was manipulated to test the effect of environmental heating or cooling on MAC. The mechanical stimulator is a form of pincer driven by compressed air, and is operated from a control panel with a manual trigger. It is capable of delivering a precise stimulus at a preselected pressure to the target site (ear pinna). The stimulus was calculated in terms of Newtons per unit surface area after calibration of the device using gram weights.